Use of cathodic protection in the manufacture of porous metal articles



3,251 758 USE OF CATHODIC PROTECTION IN THE MAN U- FACTURE F POROUS METAL ARTICLES George A. Johns and John F. Mulloy, Midland, Mich., asslgnors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Continuation of application Ser. No. 143,550, Oct. 9, 1961. This application Aug. 6, 1964, Ser. No. 387,979

4 Claims. (Cl. 204147) This application is a continuation of our application Serial No. 143,550, filed October 9, 1961, now abandoned.

This invention relates to an improved method for producing a porous metal article.

Inthe production of porous metal articles by casting metal into a salt granule filled mold and removing the salt after solidification by water leaching, corrosion prob-' lems often arise. If the metal is relatively high in the electromotive series, it may be either destroyed or converted to another form in the time required to leach the salt from the porous metal. For example, if magnesium is cast into a mold filled with sodium chloride granules and, after solidification, the casting is immersed in water, the cellular magnesium will be consumed in from 4 to 6 hours.

An object of this invention is to provide a method for leaching salt from porous metal articles without undesirable corrosion.

Other objects and advantages of the invention will be apparent during the course of the following description.

The objects of this invention are accomplished by impressing a voltage on the salt-metal casting during the leaching process so that the casting is cathodic, and providing a suitable sacrificial anode of a conducting metal.

Caustic alkali solutions are known to be beneficial as corrosion inhibitors in some cases. However, the addition of alkali to the water used to leach the'salt from a metal casting of the type here concerned is not very efiective. This is because the caustic solution is unable to penetrate the cellular structure uniformly due both to surface tension of the aqueous solution and to the evolution of hydrogen at the metal surface which blocks the l l flow of water within the porous metal.

The method of this invention combines cathodic protection with alkaline corrosion inhibition. Hydroxide is formed from the aqueous salt solution at the cathode, inhibiting corrosion of the internal pore surfaces. Thus, salt used in the casting step, such as sodium chloride and potassium chloride, must be water soluble, and have a water soluble hydroxide.

The improvement of this invention is operable regardless of the aqueous leaching method employed. Water may be used in a static (immersion) process or in a dynamic process.

The voltage to be impressed must be sufiicient to over? come the electrode potential of the porous metal, to produce the hydroxide of the salt from aqueous solution, and to overcome back and ohmic resistance in the system. Excessive voltage does not increase the protection, but has the undesirable eflect of rapidly destroying the sacrificial anode and filling the water with metallic ions from the dissolving anode.

The following example is set forth to illustrate, but

not to limit, this invention.

United States Patent O Example A casting of magnesium and sodium chloride granules was prepared by pouring molten magnesium into a salt filled mold. The casting was immersed in water and connected electrically to a piece of magnesium through a DC. power source. Voltage was then impressed across the couple so that the casting was cathodic and the magnesium was anodic. To overcome the electrode potential of magnesium, about 2.4 volts are required. In addition, about 2.3 volts are necessary to produce sodium hydroxide from an aqueous solution of sodium chloride. Thus, a minimum of about four and one-half to five volts are required to operate the system.

-Greater voltage may be necessary if the resistance of of the solution is high or the back becomes too strong. Thus, it is generally necessary to operate at a higher than minimum voltage. For leaching a magnesium-sodium chloride casting, 16 volts is generally the optimum voltage. A higher voltage may be used as long as destruction of the sacrificial anode is not so rapid that the anode is destroyed before leaching is completed.

Using a potential of 16 volts, leaching was completed in about 18 hours without serious corrosion of the porous metal. A similar casting immersed in water, but not protected according to this invention, was destroyed in about 6 hours.

Various modifications may be made in the present invention without departing from the spirit or scopethereof, and it is to be understood that we limit ourselves only as defined in the appended claims.

We claim:

1. A method of producing porous metal articles com prising casting molten metal into a mold containing salt granules, allowing the metal to solidify, connecting the salt-metal casting thus formed to a sacrificial electrode through a DC. power source, immersing the saltmetal casting in water, thereby to leach the salt from the casting, and impressing a voltage on said casting so that said casting is made cathodic to said sacrificial electrode during the leaching process, said voltage being sufficient to cause formation of hydroxide at the cathode, to overcome ohmic resistance of the system, and to overcome any back E.M.F. caused by the formation of hydrogen at the cathode.

2. The method of claim 1 wherein the metal articles are of magnesium. V

3. The method of claim 1. wherein the sacrificial electrode is magnesium.

4. The method of claim 1 wherein the salt is sodium chloride.

References Cited by the Examiner UNITED STATES PATENTS 3,108,055 10/1963 Grant 204197 OTHER REFERENCES Polonsky et al., Modern Casting, vol. 39, No. 2, February 1961, pp. AFS 65-79 [-20 (Foam Digest)].

JOHN H. MACK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner. V

T. H. TUNG, Assistant Examiner. 

1. A METHOD OF PRODUCING POROUS METAL ARTICLES COMPRISING CASTING MOLTEN METAL INTO A MOLD CONTAINING SALT GRANULES, ALLOWING THE METAL TO SOLIDIFY, CONNECTING THE SALT-METAL CASTING THUS FORMED TO A SACRIFICIAL ELECTRODE THROUGH A D.C. POWER SOURCE, IMMERSING THE SALT-METAL CASTING IN WATER, THEREBY TO LEACH THE SALT FROM THE CASTING, AND IMPRESSING A VOLTAGE ON SAID CASTING SO THAT SAID CASTING IS MADE CATHODIC TO SAID SACRIFICIAL ELECTRODE DURING THE LEACHING PROCESS, SAID VOLTAGE BEING SUFFICIENT TO CAUSE FORMATION OF HYDROXIDE AT THE CATHODE, TO OVERCOME OHMIC RESISTANCE OF THE SYSTEM, AND TO OVERCOME ANY BACK E.M.F. CAUSED BY THE FORMATION OF HYDROGEN AT THE CATHODE. 